Prior art optical rotational angle encoders typically require large, precision encoder disks mounted on a shaft so that the pattern on the disk which rotates with the shaft can be sensed and decoded in order to determine the rotational position of the shaft. These devices, however, have drawbacks. For example, they must be precisely aligned because the pattern is often sensed by a single-element photo-detector.
Other devices in the prior art measure shaft rotation by electrically sensing the rotation of an auxiliary shaft, which is coupled to the shaft to be measured by gears, pulleys or chains with sprockets. These devices have a disadvantage in that they are mechanically coupled and thus drift out of alignment over time and are further disadvantageous in that the moving mechanical linkages do not work well in extreme environments, such as in remote space vehicle applica- tions.
Hipp, U.S. Pat. No. 4,348,939 proposes to use a light source and a planar mirror both fixedly attached to a rotatable shaft so that the light from the source and reflected by the relatively fixed mirror is reflected onto a linear photodetector array which is fixed with respect to the surrounding environment. By fixing the light source and mirror with respect to each other and with respect to the movable shaft, a minimum number of photodetectors are required in order to sense a given rotational movement of the shaft. Such an arrangement is disadvantageous, however, in that it is often difficult or undersirable to fixedly mount the light source and mirror to the rotating shaft to be measured. For example, a disadvantage results if the Hipp arrangement were attempted in a system where the temperature of the rotating shaft to be measured must be maintained very low (e.g. where the shaft may be associated with cryogenically cooled apparatus in a spacecraft), in that heat generated by the light source fixedly connected to the rotating shaft may have a thermal conductive path to undesired regions.